Shielded transformer and shield therefor



Jan. 21, I941. E Q COLE SHIELDED TRANSFORMER AND SHIELD THEREFOR Filed Sept. 25, 1939 INVENTOR flan/f 0 Cale BY M l? A TTORNE Y3 Patented Jan. 21, 1941 SHIELDED TRANSFORMER AND SHIELD THEREFOR Frank 0. Cole, Detroit, Mich, assignor to The Timken-Detroit Axle Company, Detroit, Mich,

a corporation of Ohio Application September 25, 1939, Serial No. 296,523

1 Claim.-

The present invention relates to novel transformer shields for preventing high frequency disturbances in the secondary winding from afiecting the primary winding and its circuit and more B particularly to shielding the primary windings of oil burner ignition transformers to eliminate radio interference.

High frequency transformers used in the ignition circuits of oil burners are affected by the 10 high harmonics produced by the ignition spark or are in a manner to induce high frequency disturbances in the secondary winding which in turn induce similar disturbances in the primary winding and house lighting circuit to which it is 15 connected. These high frequency disturbances create considerable undesirable radio interference which it is the purpose of this invention to eliminate' To overcome this problem, the prior art has 20 proposed the use of condenser systems and grounded shields. This invention, however, is an improvement over prior shielding systems and is more effective and simple and less expensive than the prior condenser systems due to the elimina- 25 tion of the expensive choke coils and resistances which are necessary in condenser systems to obviate the so-called kick-back in the primary 35 disturbances in the secondary winding.

Still another object of the present invention resides in the provision of a shield comprising sheet copper laminated to a suitable fabric insulating material.

A further object of the present invention resides in the provision of a transformer shield made up of identical cooperating parts designed to interfit and substantially enclose the primary winding of the transformer.

45 Another object of the present invention resides in providing a transformer shield capable of adjustment to fit transformers of varying sizes.

Still other objects of the present invention will appear from the following detailed description of a preferred embodiment thereof and appended claim when read in conjunction with the attached drawing wherein:

Figure 1 is a longitudinal sectional view 55 through a conventional oil burner ignition transformer showing the present invention applied thereto.

Figure 2 is a pictorial view of one of the transformer coil units shown in Figure 1.

Figure 3 is an exploded pictorial view of the 5 novel shield of the present invention with the two parts shown in the positions they assume during assembly from opposite sides of the primary winding of the transformer.

Figure 4 is a partial pictorial view showing the upper portion of the shield of Figure 3 with its tabs bent upwardly to adapt the shield for use with a transformer core of greater thickness.

With continued reference to the drawing wherein likeparts are designated by the same reference numerals throughout the several figures, the numeral I0 indicates generally ,a conventional ignition transformer employed in domestic oil burner heating systems.

Transformer l0 includes a metallic casing II, power line conduits l2, high tension terminals I3 and a pair of transformer units I4 connected in parallel to power line conduits I2 by wires l5 and to high tension terminals l3 by means of wires i6 and I! in a manner well known in the art. Since power line conduits l2 extend a substantial distance across the casing between transformer units 14 and casing II and might be short-circuited against the casing or the transformer units, a tubular insulating, protective sleeve It is provided for housing conduits [2. A similar sleeve [9 of smaller diameter is used to protectv wire [6.

Each unit I4 is illustrated as being of the shell type. As the units Hare identical in structure, a description of one unit will be suflicient.

Unit I4 comprises a laminated core 2| of conventional construction including a central winding leg 22 and outer legs 23. Primary and sec ondary windings 24 and 25, respectively, surround winding leg 22 and fit between legs 23in well known manner. A suitable magnetic shunt 26 is disposed between coils 24 and 25. Secondary winding 25 is encased in an insulating material 21 of black varnished cambric in the usual way.

A shield 28, preferably formed by a pair of identical laminated sections 29, the edges of which are bent substantially normal to the plane of the face 30 of shield 28 to provide flanges 3| of equal depth, is provided for winding 24. Each face 30 is provided with a central substantially rectangular opening 32, designed to surround leg 22 of core 2|. Shield 28 at one side of opening 32 at substantially the midpoint thereof is provided with a slot 33 uhich extends from opening 32 to the edgeof the shield. Slot 33 is provided to prevent a short circuit-lug turn around leg 22. In addition to slot 33, shield 28 at the same side of opening 32, is preferably provided with and are employed to adapt shield. 2350:: use in varying size transformers. in this connection, tabs 3Q may be allowed to remain in the plane of faces 3H so that their edges abut the face of leg 22 or, if a larger core is tabs 5% are bent upwardly to a plane substantially normal to face 36 (as shown in Figure 4) so that the faces of tabs engage leg 22. It will thus be seen that shields 23 may be made up in standard sizes under quantity production methods and by simply bending the tabs as desired are capable of use with transformer cores built up of varying numbers of lamiuations. Suitable spring clips. 59 are employed for holding the lsminations against rela- 'tlve movement and to retain unit in proper, rigid assembled relation While shield may be manufactured in any suitable manner, the eresent invention contemplates a laminated structure rmed by laminatmg a sheet 35 of any suitable thin insulating fibre, having extremely nigh dielectric strength and good forming and bending properties, to a. thin sheet Si copper. Insulating material of the aforementioned type is well known in the trade and may be purchased on the open market under the name Fish Paper. Preferably sheets 35 having a thickness ranging between 0.00% of an inch to 0.040 of an inch are used along with copper sheets of approximately 0.305 of an inch in thickness. Fibre sheets are cemented to copper sheets ill by means of any suitable bonding agent, the usual rubber gum having been found satisfactory for this purpose. .After being formed into laminated sheets, units of suitable length are cut OE and shaped in suitable dies to provide sections 29. Each section 2% is provided with small apertures 38 adjacent the end opposite slot 33 as shown in Figures 2 and 3 for a purpose to be presently pointed out.

These sections are then placed around primary winding 26 from opposite sides to substantially house winding 24. Flanges 35 are of sufiicient length to overlap slightly, as clearly shown in Figure 2, so as to completely surround winding 24. Since sections 29 are of identical structure,

'one flange 3! of one section fits within one flange of the cooperating section while the other flange 3| fits against the outside of the other flange of the cooperating section. Due to the identity of construction of sections 29 and their manner of cooperation, it is clear that it is not necessary for the worker assembling the transformer to select an upper and lower section as would be the case if the two sections were designed so that both flanges of one were to be received between the flanges of the other.

After shield 28 is associated with winding 24, laminated core 2| is assembled with respect to windings 24 and 25 and magnetic shunt 26 inthe usual manner and insulating strips 39 are disposedbetween shield 28 and core 2| to insulate shield 28 from core 2|.

After unit M is assembled, a ground wire 4! is soldered at 52 to copper sheet or and. carried up through aperture as in lower section 29, see Figure 2, around the edge of upper section 29 and down through aperture $8 in upper section 2% and is suitably connected. to grounded case N as shown at 43 in Figure 1. Ground. connections 8%. for secondary windings 25 are also grounded at 63.

In operation, current is supplied to wires I! from the lighting circuit and passes through primary windings The current passing through windings (l t induces a high tension current in windings 2% in well known manner. The high tension current induced in secondary windlugs 25 is then transmitted through terminals 43 to the oil burner ignition electrodes (not shown) in the conventional manner.

-Durlng operation of the transformer high frequency disturbances or harmonics are induced in the secondary windings 25 by the ignition spark or are, and these disturbances will induce similar disturbances in primary windings l t and the circuit in which the primar windings are connected so as to cause objectionable radio interference unless some provision is made to effectively shield the primary windings. As these disturbances, due to their high frequency, travel through the air from the secondary winding to the primary winding, it is desirable to in some way house or shield Winding 2Q-so that these disturbances may be picked up and dissipated before they can reach the primary winding. To this and shield 28 of the present invention is provided, which completely surrounds winding 2G, and is grounded so as to intercept and transrnit these disturbances directly to the ground through wires ll and easing M9. is a result, the primary winding and its circuit are very effectively protected by the surrounding shield and the objectionable interference in the primary circuit eliminated.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefor to be considered in all respects as illustrative and-notrestrictive, the scope of the invention being indicated by the appended claim rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claim are there,- fore intended to be embraced therein.

What is claimed and desired to be secured by United States. Letters Patent is:

A shield designed to cooperate with the primary winding of an oil burner ignition transformer connected in household lighting circuits and protect said circuits from high frequency disturbances in the secondary circuit of said transformer comprising an open-ended metallic housing adapted to surround said primary winding and having a central opening for receiving sulating lining secured to said housing and insulating said housing from said winding. 

